In a dialysis machine, blood from the patient runs through tubes made of a semi-porous membrane. Outside the tubes is a sterile solution made up of water, sugars and other components. Red and white blood cells and other important blood components are too large to fit through the pores in the membranes, but urea and salt flow through membranes into the sterile solution and are removed.
Here are some interesting links:
- The Nephron Information Center: Dialysis Patient's Health Guide
- How Your Kidneys Work
- How Blood Works
- How Cells Work